It is a long-standing objective of origination colour photographic materials to maximize the overall response to light while maintaining the lowest possible granularity. Increased photographic sensitivity to light, commonly referred to as photographic speed, allows for improved images captured under low light conditions or improved details in the shadowed regions of the image. In general, the overall sensitivity to light provided by the light-sensitive silver halide emulsions in such systems is determined by the size of the emulsion grains. Larger emulsion grains capture more light. Upon development, the captured light is ultimately converted into dye deposits that constitute the reproduced image. However, the granularity exhibited by these dye deposits is directly proportional to the sizes of the silver halide grains. Thus, larger silver halide grains have higher sensitivity to light but also lead to higher granularity in the reproduced image. Therefore, it is a fundamental problem in photography to improve the light sensitivity of a silver halide element without a corresponding increase in granularity, hereinafter referred to as a ‘speed/grain effect’. Stated from another perspective, it has been a long-standing problem to provide materials which maximise the response to light of a silver halide emulsion for any given grain size.
Many different classes of heterocyclic materials are known to affect silver development in some manner and have been called, for example, antifoggants, fog restrainers, development restrainers, development inhibitors and stabilizers. Descriptions of these materials can be found in The Fundamentals of Photographic Technology, Silver Salt Photography, compiled by the Photographic Society of Japan (Corona, Ltd.), p 354; Chemistry of Photography, A. Sasai (Shashin Kogyo Shuppan Co, Ltd.), pp 168-169 and T. H. James, Ed, The Theory of the Photographic Process, 4th Edition, Macmillan Publishing Co, NY, Chapter 13, Section J. Commonly, these materials all contain an —NH or —SH group which allows them to bond or strongly adsorb to the silver surface and whose silver salts have a pKsp (−log Ksp) of more than 10, wherein Ksp is the solubility product in water at 25C.
It is well known to add these materials to silver halide emulsions in conventional colour photographic systems to limit or decrease their development. These materials are generally at least partially water-soluble or soluble in water-miscible solvents such as methanol and are added directly to silver emulsions before coating of the film or added directly to the developer solutions. It is also known to attach these types of heterocycles covalently to PUGs (photographically useful groups) so that the PUG will be held in close proximity to the silver surface see, for example, in U.S. Pat. No. 5,100,761.
U.S. Pat. Nos. 5,032,499, U.S. Pat. No. 4,837,141 and JP 62-138850 describe the use of a wide variety of photographic restrainers, including diazoles, in thermally developable light-sensitive materials. JP 10-50047 describes a wide variety of anti-silver sludging agents, including diazoles, in a non-light-sensitive cleaning film and JP 63-24255 describes a wide variety of diazoles in a colour photographic film.
US Patent Application Publication No. 2002/0045138 discloses certain imidazoles that can be used in photographic materials to improve granularity without an increase in fogging. The imidazoles are required to be N-substituted unless two adjacent substituents join to form a phenanthrene ring.
U.S. Pat. No. 5,187,054 discloses a photographic material which comprises a ballasted heterocycle, such as a benzimidazole, located in a non light-sensitive layer to reduce sludge formed during the development process but these compounds do not give a ‘speed/grain effect’.
U.S. Pat. No. 5,702,877 describes the use of ballasted benzimidazoles to improve granularity with certain pyrazolone image couplers.
U.S. Pat. No. 6,319,660 describes speed-improving compounds with, specifically, at least three heteroatoms in the ring system in a layer that contains a light-sensitive silver halide emulsion or in a non silver-containing light-insensitive layer. Among the speed-improving compounds described are purines, benzotriazoles, triazoles and thiadiazoles. Indeed generally it had been considered that for a ‘speed/grain’ effect to be observed, it was necessary for a heterocyclic compound to have at least three heteroatoms in the ring system, thereby limiting the range of suitable compounds.